Who would win?

[M.ilionario]

Okay, pay attention to this: 15+ years of military life, with constant training and exercises throughout. You did say veteran legionaries. So, take my original estimates for how long the combat would last and halve them.

Look, Jim, I said veteran legionaries not elite legionaries with 25 years of experience....
Do you need 25 years to become veteran? If you need then you're not up to the title.

@Zenodoros
I think that 30 years was the age when they started being allowed to fill civilian jobs.
But yeah,
Mental training- 7 years
Mental & Phisical- 10 years
Practice- all the way til 30 y.o.
Then at thirty they were ready for battle....

A veteran legionary would probably die in that training while less than half of the spartan teenagers suffered that fate.....
So, individually, even a spartan teenager was better than a legionary.
I think the one-on-one question is answered here

To all those believers in the all-might of the romans (like I am about spartans) I say this:
Even if there was flanking the romans would lose.
Why? Because when they had flanked the phalanx and it was forced to disband the spartans would kill the romans even if they had to do so with their bare hands, something the romans would find terrifiyng, I mean who in this world continues to fight after losing his weapons?

And you must remember: There is no cavalry going for the flanks or the rear, nor archers weakening them, it's just that, let's say 100 spartan hoplites vs 100 veteran roman legionaries.

If the romans broke the formation the it would be even worse for them:
Fencing would start....
The romans are trained to fight in close order, and so are the spartans, but the spartans, unlike all the other greeks are also trained in fencing and dueling, so.....goodbye romans.

The romans developed the auxilia for some reason don't you think?